This invention relates to franking machines and in particular to the retention of data in and retrieval of data from such machines when electrical power supply to the machine is terminated or a machine failure occurs.
Known electronic franking machines include electronic circuits, usually implemented as a microprocessor, to carry out functions of controlling operation of the machine and of carrying out accounting in respect of credit with which the franking machine is charged and use of value when mail items are franked with postage charges. Memory devices store accounting data and the stored accounting data is updated in the course of carrying out each transaction in which franking of a mail item is effected or credit is entered into the machine. For this purpose, the memory devices include a number of registers for storing the accounting data. A descending register is provided to store a value of credit entered into the machine and available for use in franking operations. An ascending register stores an accumulated value of postage charges applied to mail items when franking the mail items. An items count register stores a count of the number of mail items franked and a high items register stores a count of the number of mail items franked with a postage charge greater than a predetermined value. The accounting data stored in the memory of the franking machine provides a record of usage of the franking machine in franking mail items and hence is a record of revenue to the postal authority and of expenditure on postage charges by a user of the franking machine. Accordingly it is a requirement of the postal authorities that the accounting data is accurate and may be accurately retrieved at all times even after a failure of the franking machine. In order to ensure that the integrity of the accounting data is maintained in the event of a failure or malfunction of the memory, two memory devices are provided and each register is replicated in duplicate in each memory device. Thus four replications of each register are provided. The integrity of the accounting data stored in the memory devices is checked before and after, updating of the account records in each franking operation. This is effected by the microprocessor carrying out a check to determine that the data stored in each replication of the registers is identical. If this check indicates that an error has occurred, for example the contents of one replication of a register differ from the contents of the other replications of that register, an error flag is set and the franking machine is inhibited from use for franking operations until such time as a service call has been made and the malfunction of the franking machine has been diagnosed and corrected by an engineer authorised by the postal authority.
Errors may occur in the accounting data stored due to corruption of data already stored in the memory device or due to corruption of data when the data is being written to the memory device. Also a failure in a circuit element occur during an operation cycle and result in noncompletion of an operation in which stored accounting data is being updated.
Generally, by inspection of the contents of the registers it is possible in situations in which an error has occurred to calculate or otherwise determine the correct values of the accounting data and to correct the contents of the registers as may be necessary to ensure that the true values of accounting data are stored in all the replications of all the registers. However, particularly in the event of a substantial failure of franking machine, read out the data stored in the memory devices may be possible only by use of specialised service equipment by a service engineer.